Everyone learns during their first brewing experience that you have to carefully sanitize anything that touches your wort after the boil to avoid infection.

What happens when wort gets infected? How do you identify an infection, and what can be done about it? Can you discover it before you bottle? Does infection mean losing the batch, or can you recover from it? If you can recover, how is the flavor of the beer affected? Is infection a strong possibility if you don't sanitize, or is just paranoid beer-lovers trying to avoid the worst-case scenario: wasting good beer?

2 and 3 can sometimes be normal, depending on conditions (temperature etc.) and the gravity and fermentability of your wort. Occasionally yeast might cause odd-looking growth on the beer.

As for off flavours, the most obvious is probably a vinegar-like taste. The beer may also taste of wet cardboard, rotten milk, etc. There are a vast range of off flavours caused by infection. Read John Palmer's "How To Brew" for a comprehensive scary list.

However, I've had film moulds on the beer that don't affect flavour at all, but don't look too nice in the fermenter. Some wild yeast will cause symptom 2 and ferment all the sugars they can, so you end up with a very dry beer with no body, but without really bad flavour problems.

IMO you'll definitely get an infection sooner or later if you don't sanitise properly.

I had a batch of dunkelweizen that became infected, I think from siphoning with my mouth. The off flavor is obvious, it tastes sour. At first I thought it was completely ruined, but I held onto the batch to wait it out. Over time the flavors softened and it is actually pretty tasty now. Sort of like a dark berliner weisse.

Sanitization is definitely important, but just because you slipped up doesn't mean your batch has to be ruined. Hold onto it and try it every once and a while. You might be pleasantly surprised.

The biggest infection risk in brewing is acetobacter, for the simple reason that acetobacter is everywhere and on everything. It's in your kitchen, on your hands, and frequently on the surface of fruit fresh from the orchard. It's airborne and settles on every available surface.

Why is this a problem? Two reasons:

Acetobacter produces acetic acid, also known as vinegar. Got a vinegary tang in your brew? Chances are it's from some sort of acetobacter infection.

Acetobacter metabolizes alcohol to produce acetic acid. This is different from yeast and most other bacteria - alcohol inhibits yeast growth, but it fuels acetobacter growth.

How do you turn perfectly good brew into vinegar? Just leave the brew in an open container (cover with gauze to keep bugs out) in a warm room. Within a few days, it will start to smell vinegary, and within two weeks you will have a vat of vinegar. (ps Never taste-test homebrew vinegar - high acid content can cause serious chemical burns to your mouth!)

Sanitation is also important to reduce the risk of pathogens taking up residence in your brew and causing extreme illness or death in anyone who drinks your product. Truely toxic bacteria such as Clostridium botulinum (botulism) may be killed by boiling for extended periods, but their spores may survive boiling and begin reproduction sometime later. Sanitation with chemicals and/or high heat (above 250F) is the only way to destroy the bacteria and their spores.

Spores of clostridium botulinum are everywhere, they also happen to be inactivated easily. In a homebrew, first by oxygen presence and then by alcohol. I'd only worry about that if your batch hasn't fermented at all. I'd think it twice before drinking something that had mold growing on grain, though. Mycotoxins are scarier than botox.
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qprNov 14 '10 at 11:41

"inactivated easily" What do you mean by this? Does this mean "prevented from growing and making botulism" or does it mean "destroys the botulism toxin if any was created?" Oxygen, alcohol, and low ph all prevent c. botulinum from growing (IIRC), but does anything except heat destroy the toxin?
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rox0rNov 16 '10 at 23:52

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@roxOr I don't know the answer for sure. However, if you aren't raising the botulism bacteria where is the botulin going to come from? It's probably not very advisable to keep unfermented wort in anaerobic conditions at room temperature and then not boiling it before use. But that has nothing to do with brewing. It isn't advisable to lace the wort with botox or methanol, either.
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qprNov 19 '10 at 15:07

@qpr The thousands of homebrewers who do No Chill brewing without any known botulism cases seem to present anecdotal evidence that wort is not a good medium for botulism. Caveat: I'm a code monkey, not a scientist.
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GrahamApr 11 '11 at 13:59

So, what do you do with your vat of vinegar once you get it? You warn against taste-test... Mix with water to use?
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Cleber GoncalvesJul 22 '13 at 10:58